DEVELOPERS: add Carlo Caione for jailhouse

The jailhouse package was added in commit
ee499072 by Carlo Caione, but no entry
in the DEVELOPERS file was added. Let's fix this to ensure we have a
registered maintainer for the Jailhouse package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Carlo Caione <ccaione@baylibre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>